1. Field of the Invention
The present general inventive concept relates to an inkjet image forming apparatus, and more particularly, to an inkjet image forming apparatus that performs a high resolution using a multi-pass method.
2. Description of the Related Art
Inkjet image forming apparatuses are apparatuses for forming an image by ejecting ink to a printing medium. An inkjet image forming apparatus can be classified as a shuttle type and a line printing type. The shuttle-type inkjet image forming apparatus performs a printing job using a print head that reciprocates in a direction perpendicular to a transfer direction of a printing medium. The line printing-type inkjet image forming apparatus performs a printing job using a print head with a nozzle unit having a length that corresponds to a width of the printing medium.
Generally, a horizontal resolution of the inkjet image forming apparatus physically depends upon a gap between nozzles in the print head, that is, a nozzle pitch. A vertical resolution depends upon a transfer speed of the printing medium. Specifically, the print head of the line printing-type inkjet image forming apparatus is fixed and only the printing medium is transferred. Accordingly, when a desired resolution is greater than an actual resolution of the print head, it is difficult to provide a high-resolution printing. Since only the printing medium is transferred and the print head is fixed, the nozzles provided in the print head correspond to the transfer direction of the printing medium. FIG. 1 illustrates a print pattern formed when a nozzle unit 80 is damaged in a conventional inkjet image forming apparatus. As illustrated in FIG. 1, when a nozzle 84 is damaged, a missing line such as a white line appears on the printing medium. That is, when a part of the nozzle unit 80 of the conventional inkjet image forming apparatus is damaged, a missing line appears in the printing medium.
Referring to FIG. 1, in the conventional inkjet image forming apparatus, ink I ejected from nozzles 82 arrives at the printing medium, thereby forming an image. The conventional nozzle unit 80 is disposed perpendicular to the transfer direction of the printing medium and ejects ink droplets I to the printing medium to form an image. Therefore, when a part of the nozzles 84 are damaged, the damaged nozzles 84 (e.g., missing nozzles) do not eject ink, and missing lines such as white lines appear due to missing dots. That is, when some of the nozzles 84 are damaged, the ink droplets I are not ejected to the printing medium from the damaged nozzles 84, thereby causing the missing lines on the printing medium where the damaged nozzles 84 would have printed. This printing failure may not cause a problem in printing image data having a low printing density, however, the printing failure has a substantial negative impact on printing quality when printing image data having a high printing density.
A conventional method of compensating for deterioration in image quality caused by damage nozzles is described in U.S. Pat. No. 5,581,284. The conventional method corrects a printing failure due to failed nozzles (i.e., missing nozzles) in an inkjet image forming apparatus. Here, the failed nozzles represent nozzles which do not normally eject ink droplets such as damaged nozzles or weak nozzles. The conventional method can compensate for a black color, but cannot compensate for other colors. Since nozzles used to eject cyan ink, magenta ink, and yellow ink do not work when printing the black color, a process black can be formed using these different color inks. However, when an image having various colors is printed (i.e., when the nozzles for cyan ink, magenta ink, and yellow ink are used to eject ink), the compensation cannot be performed. In addition, when one nozzle is damaged, the missing lines are corrected using another color such as red (yellow+magenta), green (cyan+yellow), and blue (cyan+magenta), which are contrasted with the process black such that the printed image quality is negatively affected. Furthermore, since color ink or mixed ink should be ejected so as to compensate for the black color, an amount of the color ink to be used is increased, thereby shortening a lifespan of an ink cartridge.
According to the conventional nozzle unit 80 and the conventional method described above, it is not easy to perform a printing job with a resolution higher than the actual resolution of the print head. In addition, the printing failure due to the failed nozzles may cause a problem in increasing a printing speed and an image quality of the inkjet image forming apparatus.